1. Field of the Invention
The present invention relates to a backprojection and/or projection screen. It relates more particularly to a screen suitable for allowing an image to be displayed without any defect on both faces of the screen, this image possibly coming, for example, from a video projector or more generally from a light source positioned so as to face one of the faces of the screen.
2. Description of the Background
For the sake of understanding, the projection screen according to the invention has two faces, namely the main face of the screen on which the image coming from the light source is projected, which main face is generally positioned in the same region in space as the light source, and the opposite face of the screen on which the image projected on the main face appears when viewed in transmission. A backprojection screen has a main face and an opposite face possessing the same characteristics as the abovementioned projection screen but is distinguished therefrom by the fact that the user and the light source are not located in the same region in space but are on either side of the screen.
For the purposes of the present invention, the term “screen” will be used to encompass both a projection screen and a backprojection screen.
Several types of backprojection screen families for displaying such an image are known. Thus, plastic screens or diffusers are known, these rigid and translucent screens having a plastic matrix constituting the framework of the screen into which a plurality of metal oxide (for example SiO2) particles are incorporated. These screens allow an image to be correctly viewed without any optical defect at a viewing angle possibly up to 180° only on one of the faces of the screen, the image appearing when viewed in transmission on the other face of the screen being of lower optical quality (presence of blurring). In addition, plastic backprojection screens are sensitive to ultraviolet and infrared radiation and to moisture, thereby limiting their lifetime and the perpetuity of their optical quality.
Also known are screens that can be unwound from a reel fastened to a support. They are also made of plastic, but are not translucent. They operate only in reflection and not in transmission.
Another screen technology was developed from diffusers that operate using the technology of liquid crystals. The main advantage of this type of screen lies, on the one hand, in their small thickness (of around 20 μm) and, on the other hand, most particularly, in the fact that, under the action of an electric field, they become transparent and no longer diffusing, their use in backprojection being however only a secondary use.
However, as drawbacks, the viewing angle is very limited (the viewing angle is around thirty degrees or so) and the image that appears on the opposite face is of poor optical quality, despite the small thickness of the screen (presence of blurring).
Document U.S. Pat. No. 5,870,224 discloses a structure of projection screen provided, on the one hand, on its main face, with a plurality of optical devices (cylindrical microlenses and Fresnel lenses) and, on the other hand, on its opposite face, a plurality of orifices (transparent and slightly diffusing regions) that are judiciously positioned relative to the respective optical foci of the said cylindrical lenses, allowing the projected image to be displayed.
This screen structure is particularly expensive, the viewing angle is also small (about 35°) and this device is not conducive to direct projection (view in reflection)
Finally, a last backprojection screen family consists of holographic diffusers.
The main advantage of this technology lies in the transparency of the screen and the adjustment of the viewing angle. However, here again, this screen structure is expensive, thereby confining it to small-format screens. The presence of blurring on the opposite face of the screen is also noted, despite the transparency of the screen.